With optical lenses, a need very often arises for applying a protective coating which is light-permeable and yet is reflection-reducing. This applies for camera lenses as well as also for lenses of eye glasses. In the case of inept or careless handling by the user eyeglasses can be subjected to extreme stress which causes scratching not only of synthetic glasses but also of silicate glasses. Sharp objects are typically responsible when drawn across the lens surface under pressure. Examples include dust or sand with sharp edges in a polishing cloth or in a case for storing the glasses, rough tissue used to wipe them.
Because they combine the property of low weight and greater resistance to breakage, with the possibility of individual coloring, synthetic glasses are used increasingly more often. However, they have the serious disadvantage that their surfaces, which are considerably softer compared to silicate glasses are very susceptible to mechanical damage.
Duroplastics which comprise macromolecules chemically closely enmeshed with one another are used widely to make synthetic lenses. They are most often very brittle at room temperatures. In addition, they are temperature stable, not weldable, insoluble, and only weakly swellable. One duroplastic preferred in lens systems used for eyeglasses is CR 39 which is a diallyldiethylene glycolcarbonate. Only very recently, apart from these synthetic materials used nearly exclusively in lens systems for glasses, other synthetic materials such as polymethylmethacrylate (PMMA), polystyrene (PS), and polycarbonate (PC) have also been employed.
If, for example, a CR 39 synthetic is to be provided with an appropriate protective covering the problems of detachment of the protective layer from the lens body, the difference in thermal expansion between the protective layer and the body, and in many cases the low temperature stability of the protective coating, must be solved.
With silicate coatings, the substrate is heated to a high temperature of approximately 300.degree. C. whereby sufficient energy is available of the coating molecules applied in a vapor deposition process to generate defect-free dense layers. In contrast, when producing a synthetic layer, the vapor deposition as a rule must be carried out at low temperature.
In order to make available the energy required in this case, the grown layers are bombarded with ions of an inert gas. Additional ionization of vapor particles and a reactive gas reinforces the densification process in layer-forming condensation.
Apart from these so-called ion-assisted vapor deposition processes (IAD), so-called plasma polymerization is also known in which during the layer formation the properties of the layer can be changed continuously. The resulting properties are adapted to the synthetic surface from the aspect of the chemical structure and form on the lens surface a glass-like structure which has very high mechanical abrasion resistance.
A process for the production of transparent protective coatings comprising silicon compounds is already known and used in the coating of synthetic substrates (See DE-A-3 624 467 and EP-A-0 254 205). In this process, a chemical vapor deposition takes place under the effect of a plasma (plasma chemical vapor deposition) onto a polymerizable monomeric organic compound from the group of the siloxanes and silazanes wherein to the polymerization process oxygen is supplied in excess. Plasma is generated by means of high frequency current between two electrodes of which the one has the function of a cathode and is connected with the substrates. Before the coating proper, the substrates are exposed in an atmosphere comprising a noble gas to an ion bombardment by glow discharge in the presence of the organic compounds.
In another known process of plasma-enhanced coating of a substrate with a polymerizable silicon-comprising monomer the monomers are restricted to silanes, silazanes or disilazanes, and the plasma coating is carried out until a particular Taber wear index is attained (See EP-A-0 252 870).
A device is also known (See DE-C-3 931 713) with which optical lenses can be coated on both sides in a plasma-enhanced process. This device comprises two electrodes between which are disposed holding elements for the work pieces to be coated. The holding elements therein are at a defined electrical potential.
A similar process is known (per EP-A-0 403 985) for the pretreatment of transparent synthetic substrates intended for vacuum coating. It has been found that through plasma bombardment of the substrate surface this surface is changed in such a way that the subsequent layer can be applied with a high degree of adhesive strength.
In a further device for the coating of substrates a vacuum chamber is provided with a substrate carrier disposed in it and having a plasma generator, a magnet and an electron emitter. Also, in the vacuum chamber a device is provided for the generation of atoms, molecules or clusters of the materials for the generation of the layer on the substrates, and is located immediately next to the plasma generator and opposite the substrates (See EP-90123712.3, K. Matl, W. Klug, A. Zoller; "Ion assisted deposition with a new plasma source," Paper presented at the Sec. PSE Conf., Garmisch-Partenkirchen 1990). One advantage of this device resides in that, in contrast to earlier devices of the ion-assisted deposition (IAD) type, it can act upon substrate holders having a diameter of approximately 1 m with high plasma density.
Moreover, antireflection coatings are known which are applied onto synthetic lenses and comprise, for example, two layers of which the first layer is a SiO.sub.x layer and the second layer is a SiO layer (per DE-OS 27 38 044, FIG. 1A).
Further, antireflection coatings are known comprising four or more discrete layers wherein, for example, beginning at the substrate, the layer sequence is as follows: SiO, SiO.sub.2, CeO.sub.2, SiO.sub.2, CeO.sub.2, SiO.sub.2, CeO.sub.2 (See DE-OS 38 18 341).
Lastly, a process for the production of synthetic objects with hard coatings is also known, in which a layer based on silicon is disposed on a foundation material and a SiO.sub.2 film is applied onto this layer (See EP-A-0 266 225). The SiO.sub.2 layer is applied by means of a vacuum vapor deposition process, preferably in an ion-plating process.